1610. Is the Surgical Invasiveness Index valid in Minimally Invasive Spine Surgery?

Authors: Avani Vaishnav; Jung Mok, BS; Catherine Gang, MPH; Steven McAnany, MD; Sravisht Iyer, MD; Todd Albert, MD; Sheeraz Qureshi, MD, MBA (New York, NY)

Introduction:
The Surgical Invasiveness Index(SII) is often used to determine the magnitude of a surgical procedure i.e. ‘invasiveness’, and thus assess the associated risks and predict recovery. However, in the cohort used to create this index, only 6.7% were minimally invasive procedures. Thus, it is not known whether this index is valid in minimally invasive spine surgery(MISS).

Purpose:
To assess the validity of the Surgical Invasiveness Index in MISS.

Method: 
SII was calculated for 386 patients who underwent MISS. Linear regressions were run with procedure time(in minutes) and estimated blood loss(EBL, in ml) as dependent variables.

Results: 
SII ranged from 1 to 29 (median:5;IQR:2-8). The procedure time ranged from 17 to 418 minutes (median:65;IQR:48.75-95]. Although EBL ranged from 10 ml to 600 ml (median:25;IQR:25-25), 81.6% of patients had EBL<25ml and 97.7% had EBL<100ml.

The regression analysis for procedure time showed a strong statistically significant association (r2=0.695;p<0.0001), indicating that the SII explains 69.5% of the variability in procedure time. In contrast, the regression analysis for EBL showed a weak association (r2=0.161;p<0.0001), indicating that the SII explains only 16.1% of the variability in EBL.

Conclusion: 
The results of our study show that the SII explains almost 70% of the variability in duration of the surgical procedure for MIS procedures. However, it does not adequately explain the variability in EBL. Given the low blood loss in MIS surgery, the SII may not appropriately reflect surgical stress in these patients.  Larger studies are warranted to assess the validity of the SII in MIS procedures and potentially develop a MIS-specific Invasiveness Index. A more refined and comprehensive index, which accounts for the minimally invasive approach may more accurately reflect the magnitude of surgery in these patients and thus better predict surgical outcomes in terms blood loss, risk of complications, post-operative recovery and functional outcomes.